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Surface modification of carbon nanotubes by anionic approach
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ScienceDirect Surface modification of carbon nanotubes by anionic approach Kaoru Adachi and Yasuhisa Tsukahara Address Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Corresponding author: Tsukahara, Yasuhisa ([email protected])
Current Opinion in Chemical Engineering 2016, 11:106–113 This review comes from a themed issue on Materials engineering Edited by Thein Kyu and Jai A Sekhar
http://dx.doi.org/10.1016/j.coche.2016.01.002 2211-3398/Published by Elsevier Ltd.
Introduction Recently, carbon nanotubes (CNTs) have been investigated and identified for applications in new classes of high performance composites because of their intriguing properties, including extremely high mechanical strength, high thermal stabilities, and electrical and thermal conductivities [1,2,3–8,9,10]. In general, finer dispersions of CNTs in polymeric media hold promise for high performance composites. However, CNTs tend to aggregate and form bundles with strong van der Waals interactions on account of the large surface area they possess. This can lead to phase separation of the CNTs from a polymeric phase that prevents homogeneous mixing with one another. In spite of the expected promising properties of CNTs, their solubility in any common media, including organic solvents, water, and polymeric matrices, is quite poor. Therefore, surface modification methods have been proposed from the initial stages of CNT development to overcome this obstacle. Despite recent interest from a number of research groups focusing on CNTs, taking advantage of the high performance capabilities of CNT composites remains to be fully realized and an ongoing challenge. Since CNTs have only limited chemical reactivity, surface modification of CNTs involves both non-covalent physical modifications and covalent modification by chemical methods. In the former non-covalent approach, CNTs can be dispersed in a variety of media in the presence of an appropriate dispersant bearing both affinity moieties for the CNT and the media [11–17]. The dispersants, which include low molecular weight compounds and polymeric ones, cover the surface of the Current Opinion in Chemical Engineering 2016, 11:106–113
CNTs non-covalently to alleviate aggregation. This type of modification excludes chemical decomposition of p conjugated hexagonal rings of pristine CNTs. Hence, the modification method is effective for applications that require increased electrical conductivity. In a latter approach, despite the limited chemical reactivity of the CNT surface, covalent modifications have been developed, including methods such as acid oxidation [2,7,8,9,18–20], radical [7,8,20–23] or anionic surface modification [7,8,9,24], or modification by other organic reactions [7,8,9,25–27]. Acid oxidation is one of the most basic techniques for covalent CNT modification, whereby carboxyl or hydroxyl groups on the surface are generated, allowing for further modifications, such as polymer grafting, as a result of the versatility these functional groups have for organic reactions. Radical addition is another technique that can be used for covalent modification, whereby the grafting of polymers to CNTs can be achieved by both coupling the functional groups on CNTs with corresponding groups on polymers (grafting-onto), and polymerization of monomers from initiating sites on CNTs (grafting-from). Recently, in addition to those classical surface modification methods, anionic approaches using organometallic compounds have gained increased attention. This approach leads to covalent modifications by the nucleophilic addition onto the p conjugated hexagonal rings of the CNTs. In this short review, we focus on the anionic approach for CNT modifications with low molecular weight compounds, followed by modifications with polymeric compounds.
Covalent modification with alkyllithium Direct lithiation of CNTs without further purification was reported by Viswanathan et al. in 2003 [24]. They developed a simple approach for surface lithiation of single walled carbon nanotubes (SWCNTs) with sec-BuLi by sonication in cyclohexane. SWCNTs were exfoliated by this reaction and the lithiated product showed excellent dispersion with long-term stability in solution due to the electrostatic repulsion of the CNT surfaces. Moreover, the lithiated SWCNTs had the ability to initiate polymerization of styrene monomer anionically by simply adding the monomer, which resulted in CNT/polymer composites. This reaction was confirmed by ab initio quantum chemistry calculations [28]. The sec-Bu unit is covalently connected to the nanotube by nucleophilic addition and the obtained negative charge is delocalized on several carbon atoms. The calculation also indicated www.sciencedirect.com
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that the CNT anion has a potential strong enough to attack the propylene oxide nucleophilically to open the epoxy ring. Hirsch and coworkers also reported alkylation with alkyllithiums and various Grignard reagents [29,30]. They described the reactivity differences between metallic tubes and semiconducting tubes, and observed corresponding functional group on the CNTs by scanning tunneling microscopy (STM) analysis (Figure 1).
indicate a single electron transfer from the reduced CNT to the halide, which generates corresponding radicals, and a subsequent radical addition onto the CNT surface. Modification reactions of CNTs with various carbonyl compounds or functionalized alkyl/aryl iodides under these conditions were developed [37,38].
Functionalization with organic metal compounds
CNT/polymer composites construction of the butyl modified multi-walled carbon nanotubes (MWCNTs) and polystyrene have been examined [31]. Dispersion of the functionalized MWCNTs in a polystyrene matrix was clearly improved relative to pristine MWCNTs as a result of presence of the surface butyl groups on the CNTs. The composites obtained using modified MWCNTs showed higher mechanical properties than those using pristine MWCNTs. This result indicates the dispersion of CNTs into a polymer matrix enhances mechanical properties of the composites. Interestingly, although the defects of MWCNTs increases with repeated butyl modifications, which was calculated from the intensity ratio of the G-band and to the D-band (G/D ratio) from Raman spectra, the mechanical properties of the obtained composites actually improved at 0.125% loading of the MWCNTs.
After the report of CNT lithiation, two pathways for surface functionalization of CNTs via lithiation were investigated. One is the reaction between CNT anions and the appropriate electrophiles, such as epoxides and alkyl halides. The other is the direct functionalization of CNT with metallated molecules. In the former case, we have reported the introduction of hydroxyl groups onto MWCNTs by lithiation with sec-BuLi and the subsequent ring opening reaction with ethylene oxide or ethylene glycol diglycidyl ether [39]. The obtained hydroxylated MWCNTs showed good dispersibility in protic solvents because of the hydrophilic hydroxyl groups on the MWCNTs surface. The product can also be dispersed in a poly(vinyl alcohol) matrix and the composites showed both high tensile strength and high elongation at the breaking point. Functionalized SWCNTs having two different alkyl groups have been synthesized, with the approach consisting of first alkylation by lithiation of SWCNTs with alkyllithium and then subsequent alkylation by addition of alkyl halides into the lithiated product [40]. Raman spectroscopy is a powerful tool for analyzing the reactions and calculating the functional group coverage on the basis of G/D ratio [41]. Amino-fuctionalized MWCNTs were prepared by the lithiation of MWCNTs with n-BuLi in diethylether, followed by electrophilic attack of 2-diethylaminoethyl bromide [42]. The high resolution transmission electron microscopy (HRTEM) images of the obtained aminofunctionalized MWCNTs by lithiation and electrophilic attack, compared to that prepared by conventional approach using acid treatment and amidation, showed significant differences in the MWCNT structure, with less
Birch type reactions are a familiar way of surface modification for CNTs. Hydrogenation by this approach was first developed by Pekker and co-workers [32]. They reported on the reaction of CNTs with metal lithium in liquid ammonia and subsequent quenching with methanol, resulting in hydrogenated CNTs. In this system, an anionic mechanism was proposed. With a similar method, Billups and co-workers developed alkylation and arylation through the reaction between SWCNTs and alkali metals, and the subsequent addition of appropriate alkyl/ aryl halides in liquid ammonia to functionalize SWCNTs [33–36]. Analyzing byproducts of the reaction suggests the existence of radical species. The results obtained Figure 1
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Lithiation of CNTs with sec-BuLi by nucleophilic addition.
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Various organic functionalizations of CNTs via lithiation: introduction of (a) hydroxyl, (b) amino, (c) carboxyl, and (d) epoxy groups.
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Surface modification of carbon nanotubes Adachi and Tsukahara 109
damage to CNT backbones via the lithiation approach. This result indicates that anionic functionalization is a relatively less harsh method to help preserve the desired pristine CNT properties. They also reported catalytic activity of the amino functionalized MWCNTs for transesterification of triglycerides [43,44]. Carboxyl functionalization was also reported through lithiation of CNTs. Since carbanions are known to react with carbon dioxide to form carboxylate anions, this reaction was applied to CNT anions, which were obtained by lithiation of SWCNTs in tetrahydrofuran (THF) [45]. The obtained carboxylated SWCNTs were highly dispersible in aqueous media at concentrations up to 0.5 mg/ mL, indicating a hydrophilic surface. Other functional groups were also introduced by using the lithiated CNTs, such as ester groups that have been introduced onto SWCNT surfaces by the addition of haloformates into n-BuLi modified SWCNTs. These ester groups were further amidated by alkylamines [46]. In another example, epoxide functionalized SWCNTs were reported by utilizing lithiation and subsequent reaction with epichlorohydrin [47]. These epoxy groups on the CNTs were
further reacted with aminated poly(phenylene sulfide) to yield covalently bonded polymeric composites. Various properties of these composites were compared with composites prepared with acid modified SWCNTs. Those prepared from epoxy modified SCNTs possessed higher electrical conductivity, indicating less damage at the hexagonal aromatic rings compared to those prepared by modification with acid treatment. The latter direct functionalization approach of SWCNTs by metallated compounds involves the modification with lithium alkylides and aromatic acetylides [48]. These organolithium compounds have relatively low reactivity compared with alkyllithium compounds, although nucleophilic addition reactions do proceed at elevated temperature. Such addition reactions by compounds having relatively low nucleophilicity expand the possibilities of functionalization of CNTs by nucleophilic attack (Figure 2).
Grafting-onto surface modification As described above, CNTs have the potential to react with various carbanions, thus enabling the preparation of
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Polymer surface modifications of CNTs by grafting-onto approach: modification by (a) nucleophilic addition of polystyryl anion onto CNT and (b) quenching of lithiated CNT with chlorinated polypropylene. www.sciencedirect.com
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CNT/polymer composites via polymer grafting. Since anionic living polymerization involves a highly reactive carbanion at the end of the polymer chain, the carbanions can be introduced directly onto the surface of pristine
CNTs anionically. Mountrichas et al. prepared polystyrene grafted CNTs with a variety of chain lengths by the nucleophilic addition of polystyryl anion onto CNTs in benzene [49]. The obtained products showed solubility in
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Polymer surface modifications of CNTs by grafting-from approach: grafting of (a) polystyrene and (b) poly(tert-butyl acrylate-b-methyl methacrylate) by anionic polymerization. Current Opinion in Chemical Engineering 2016, 11:106–113
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Surface modification of carbon nanotubes Adachi and Tsukahara 111
polystyrene and are soluble in organic media such as toluene, THF, chloroform, and DMF. With this method, up to 82 wt% of the polystyrene was introduced onto the MWCNT surface. A similar approach enables one to graft poly(1,3-cyclohexadiene) onto SWCNT surfaces [50]. The reaction between SWCNTs and poly(1,3-cyclohexadienyl) lithium in toluene at room temperature under sonication leads to high solubility of the CNT in organic solvents by the covalent polymer modification. In this case, 2 mol% of carbon atoms on SWCNTs were reacted with the anion even as an excess of anions existed in the reaction mixture. However, the amount of grafted polymer was controlled by the molecular weight of the living polymer. In yet another case, 4-vinylpyridine monomer was anionically polymerized from n-BuLi and added to MWCNTs to prepare poly(4-vinylpyridine) grafted CNTs. The grafted polymer was able to then be converted into a polymeric ionic liquid by quaternization with bromoethane [51]. In the above cases, CNTs act as the terminating agent for living anionic polymerization. Besides that, lithiated CNTs can be used for polymer grafting by the grafting-onto approach. In this case, polymers having quenching agents on their chains, such as halogenated groups, on their chains are good candidates for grafting. Blake et al. synthesized polypropylene-grafted MWCNTs by quenching the lithiated CNTs with chlorinated polypropylene [52]. The MWCNTs obtained showed promise as high performance fillers to reinforce chlorinated polypropylene (Figure 3).
Grafting-from surface modification Carbanions obtained from CNTs by nucleophilic attack of alkyl anions are strong enough for the initiation of anionic polymerizations of various monomers, so that anionic polymerizations have been developed for covalent surface modification (grafting-from approach). As mentioned above, Viswanathan et al. reported on the polymerization of styrene from lithiated SWCNTs at the same time of the first report of CNT lithiations [24]. It is a simple method comprising lithiation of SWCNTs, followed by the addition of styrene monomer into the solution. Since anionic polymerization proceeds as a living reaction, controlling molecular weight of the grafting polymer is possible. The polystyrene modified SWCNTs are soluble in organic solvents because of the grafted polymer chains. Polyisoprene was also grafted by a similar procedure [53]. CNT/polyisoprene nanocomposites containing polyisoprene grafted SWCNTs were prepared by the polymerization of isoprene in the presence of in situ generated lithiated SWCNTs. The nanocomposites showed good solubility in organic solvents, and higher glass transition temperatures and thermal stability than that of pure polyisoprene. Polymerization of tert-butyl acrylate in THF has been initiated from the surface of SWCNTs and subsequent block copolymerization of www.sciencedirect.com
methyl methacrylate has been carried out [54]. It is noteworthy that methyl methacrylate by itself is not polymerized from the lithiated SWCNTs under the same conditions. Heteroatom-containing monomers have also been polymerized anionically for practical applications. N-Vinylcarbazol can be polymerized from the SWCNT anion, resulting in polyvinylcarbazol (PVK) grafted SWCNTs [55]. It has not only good solubility in organic solvents, but also a unique absorption in the 350–500 nm range due to a charge transfer between the electron withdrawing SWCNTs and the electron donating PVK. It is also interesting to note that it showed better optical limiting performance than that of pristine SWCNTs (Figure 4).
Conclusions After the introduction of the anionic surface modification method for CNTs, a large number of researchers have been investigating the solubilization and functionalization of CNTs, as well as the preparation of CNT/polymer composites. Just recently, it was discovered that anionic surface modification has several advantages in the variety of the reactions and the covalent modification of CNTs cause less damage on the structure than acid oxidation. As described above, most of the cases improved the solubility of the CNTs in various media to a certain extent. However, further improvement in their properties remains an ongoing challenge. Further practical applications based on the advantages of using the anionic surface modification are expected to be the main focus of research in the near term in order to make use of the unique and valuable properties of CNTs.
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ScienceDirect Surface modification of carbon nanotubes by anionic approach Kaoru Adachi and Yasuhisa Tsukahara Address Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Corresponding author: Tsukahara, Yasuhisa ([email protected])
Current Opinion in Chemical Engineering 2016, 11:106–113 This review comes from a themed issue on Materials engineering Edited by Thein Kyu and Jai A Sekhar
http://dx.doi.org/10.1016/j.coche.2016.01.002 2211-3398/Published by Elsevier Ltd.
Introduction Recently, carbon nanotubes (CNTs) have been investigated and identified for applications in new classes of high performance composites because of their intriguing properties, including extremely high mechanical strength, high thermal stabilities, and electrical and thermal conductivities [1,2,3–8,9,10]. In general, finer dispersions of CNTs in polymeric media hold promise for high performance composites. However, CNTs tend to aggregate and form bundles with strong van der Waals interactions on account of the large surface area they possess. This can lead to phase separation of the CNTs from a polymeric phase that prevents homogeneous mixing with one another. In spite of the expected promising properties of CNTs, their solubility in any common media, including organic solvents, water, and polymeric matrices, is quite poor. Therefore, surface modification methods have been proposed from the initial stages of CNT development to overcome this obstacle. Despite recent interest from a number of research groups focusing on CNTs, taking advantage of the high performance capabilities of CNT composites remains to be fully realized and an ongoing challenge. Since CNTs have only limited chemical reactivity, surface modification of CNTs involves both non-covalent physical modifications and covalent modification by chemical methods. In the former non-covalent approach, CNTs can be dispersed in a variety of media in the presence of an appropriate dispersant bearing both affinity moieties for the CNT and the media [11–17]. The dispersants, which include low molecular weight compounds and polymeric ones, cover the surface of the Current Opinion in Chemical Engineering 2016, 11:106–113
CNTs non-covalently to alleviate aggregation. This type of modification excludes chemical decomposition of p conjugated hexagonal rings of pristine CNTs. Hence, the modification method is effective for applications that require increased electrical conductivity. In a latter approach, despite the limited chemical reactivity of the CNT surface, covalent modifications have been developed, including methods such as acid oxidation [2,7,8,9,18–20], radical [7,8,20–23] or anionic surface modification [7,8,9,24], or modification by other organic reactions [7,8,9,25–27]. Acid oxidation is one of the most basic techniques for covalent CNT modification, whereby carboxyl or hydroxyl groups on the surface are generated, allowing for further modifications, such as polymer grafting, as a result of the versatility these functional groups have for organic reactions. Radical addition is another technique that can be used for covalent modification, whereby the grafting of polymers to CNTs can be achieved by both coupling the functional groups on CNTs with corresponding groups on polymers (grafting-onto), and polymerization of monomers from initiating sites on CNTs (grafting-from). Recently, in addition to those classical surface modification methods, anionic approaches using organometallic compounds have gained increased attention. This approach leads to covalent modifications by the nucleophilic addition onto the p conjugated hexagonal rings of the CNTs. In this short review, we focus on the anionic approach for CNT modifications with low molecular weight compounds, followed by modifications with polymeric compounds.
Covalent modification with alkyllithium Direct lithiation of CNTs without further purification was reported by Viswanathan et al. in 2003 [24]. They developed a simple approach for surface lithiation of single walled carbon nanotubes (SWCNTs) with sec-BuLi by sonication in cyclohexane. SWCNTs were exfoliated by this reaction and the lithiated product showed excellent dispersion with long-term stability in solution due to the electrostatic repulsion of the CNT surfaces. Moreover, the lithiated SWCNTs had the ability to initiate polymerization of styrene monomer anionically by simply adding the monomer, which resulted in CNT/polymer composites. This reaction was confirmed by ab initio quantum chemistry calculations [28]. The sec-Bu unit is covalently connected to the nanotube by nucleophilic addition and the obtained negative charge is delocalized on several carbon atoms. The calculation also indicated www.sciencedirect.com
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that the CNT anion has a potential strong enough to attack the propylene oxide nucleophilically to open the epoxy ring. Hirsch and coworkers also reported alkylation with alkyllithiums and various Grignard reagents [29,30]. They described the reactivity differences between metallic tubes and semiconducting tubes, and observed corresponding functional group on the CNTs by scanning tunneling microscopy (STM) analysis (Figure 1).
indicate a single electron transfer from the reduced CNT to the halide, which generates corresponding radicals, and a subsequent radical addition onto the CNT surface. Modification reactions of CNTs with various carbonyl compounds or functionalized alkyl/aryl iodides under these conditions were developed [37,38].
Functionalization with organic metal compounds
CNT/polymer composites construction of the butyl modified multi-walled carbon nanotubes (MWCNTs) and polystyrene have been examined [31]. Dispersion of the functionalized MWCNTs in a polystyrene matrix was clearly improved relative to pristine MWCNTs as a result of presence of the surface butyl groups on the CNTs. The composites obtained using modified MWCNTs showed higher mechanical properties than those using pristine MWCNTs. This result indicates the dispersion of CNTs into a polymer matrix enhances mechanical properties of the composites. Interestingly, although the defects of MWCNTs increases with repeated butyl modifications, which was calculated from the intensity ratio of the G-band and to the D-band (G/D ratio) from Raman spectra, the mechanical properties of the obtained composites actually improved at 0.125% loading of the MWCNTs.
After the report of CNT lithiation, two pathways for surface functionalization of CNTs via lithiation were investigated. One is the reaction between CNT anions and the appropriate electrophiles, such as epoxides and alkyl halides. The other is the direct functionalization of CNT with metallated molecules. In the former case, we have reported the introduction of hydroxyl groups onto MWCNTs by lithiation with sec-BuLi and the subsequent ring opening reaction with ethylene oxide or ethylene glycol diglycidyl ether [39]. The obtained hydroxylated MWCNTs showed good dispersibility in protic solvents because of the hydrophilic hydroxyl groups on the MWCNTs surface. The product can also be dispersed in a poly(vinyl alcohol) matrix and the composites showed both high tensile strength and high elongation at the breaking point. Functionalized SWCNTs having two different alkyl groups have been synthesized, with the approach consisting of first alkylation by lithiation of SWCNTs with alkyllithium and then subsequent alkylation by addition of alkyl halides into the lithiated product [40]. Raman spectroscopy is a powerful tool for analyzing the reactions and calculating the functional group coverage on the basis of G/D ratio [41]. Amino-fuctionalized MWCNTs were prepared by the lithiation of MWCNTs with n-BuLi in diethylether, followed by electrophilic attack of 2-diethylaminoethyl bromide [42]. The high resolution transmission electron microscopy (HRTEM) images of the obtained aminofunctionalized MWCNTs by lithiation and electrophilic attack, compared to that prepared by conventional approach using acid treatment and amidation, showed significant differences in the MWCNT structure, with less
Birch type reactions are a familiar way of surface modification for CNTs. Hydrogenation by this approach was first developed by Pekker and co-workers [32]. They reported on the reaction of CNTs with metal lithium in liquid ammonia and subsequent quenching with methanol, resulting in hydrogenated CNTs. In this system, an anionic mechanism was proposed. With a similar method, Billups and co-workers developed alkylation and arylation through the reaction between SWCNTs and alkali metals, and the subsequent addition of appropriate alkyl/ aryl halides in liquid ammonia to functionalize SWCNTs [33–36]. Analyzing byproducts of the reaction suggests the existence of radical species. The results obtained Figure 1
Li Li
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Lithiation of CNTs with sec-BuLi by nucleophilic addition.
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108 Materials engineering
Figure 2
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Various organic functionalizations of CNTs via lithiation: introduction of (a) hydroxyl, (b) amino, (c) carboxyl, and (d) epoxy groups.
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Surface modification of carbon nanotubes Adachi and Tsukahara 109
damage to CNT backbones via the lithiation approach. This result indicates that anionic functionalization is a relatively less harsh method to help preserve the desired pristine CNT properties. They also reported catalytic activity of the amino functionalized MWCNTs for transesterification of triglycerides [43,44]. Carboxyl functionalization was also reported through lithiation of CNTs. Since carbanions are known to react with carbon dioxide to form carboxylate anions, this reaction was applied to CNT anions, which were obtained by lithiation of SWCNTs in tetrahydrofuran (THF) [45]. The obtained carboxylated SWCNTs were highly dispersible in aqueous media at concentrations up to 0.5 mg/ mL, indicating a hydrophilic surface. Other functional groups were also introduced by using the lithiated CNTs, such as ester groups that have been introduced onto SWCNT surfaces by the addition of haloformates into n-BuLi modified SWCNTs. These ester groups were further amidated by alkylamines [46]. In another example, epoxide functionalized SWCNTs were reported by utilizing lithiation and subsequent reaction with epichlorohydrin [47]. These epoxy groups on the CNTs were
further reacted with aminated poly(phenylene sulfide) to yield covalently bonded polymeric composites. Various properties of these composites were compared with composites prepared with acid modified SWCNTs. Those prepared from epoxy modified SCNTs possessed higher electrical conductivity, indicating less damage at the hexagonal aromatic rings compared to those prepared by modification with acid treatment. The latter direct functionalization approach of SWCNTs by metallated compounds involves the modification with lithium alkylides and aromatic acetylides [48]. These organolithium compounds have relatively low reactivity compared with alkyllithium compounds, although nucleophilic addition reactions do proceed at elevated temperature. Such addition reactions by compounds having relatively low nucleophilicity expand the possibilities of functionalization of CNTs by nucleophilic attack (Figure 2).
Grafting-onto surface modification As described above, CNTs have the potential to react with various carbanions, thus enabling the preparation of
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Polymer surface modifications of CNTs by grafting-onto approach: modification by (a) nucleophilic addition of polystyryl anion onto CNT and (b) quenching of lithiated CNT with chlorinated polypropylene. www.sciencedirect.com
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CNT/polymer composites via polymer grafting. Since anionic living polymerization involves a highly reactive carbanion at the end of the polymer chain, the carbanions can be introduced directly onto the surface of pristine
CNTs anionically. Mountrichas et al. prepared polystyrene grafted CNTs with a variety of chain lengths by the nucleophilic addition of polystyryl anion onto CNTs in benzene [49]. The obtained products showed solubility in
Figure 4
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Polymer surface modifications of CNTs by grafting-from approach: grafting of (a) polystyrene and (b) poly(tert-butyl acrylate-b-methyl methacrylate) by anionic polymerization. Current Opinion in Chemical Engineering 2016, 11:106–113
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Surface modification of carbon nanotubes Adachi and Tsukahara 111
polystyrene and are soluble in organic media such as toluene, THF, chloroform, and DMF. With this method, up to 82 wt% of the polystyrene was introduced onto the MWCNT surface. A similar approach enables one to graft poly(1,3-cyclohexadiene) onto SWCNT surfaces [50]. The reaction between SWCNTs and poly(1,3-cyclohexadienyl) lithium in toluene at room temperature under sonication leads to high solubility of the CNT in organic solvents by the covalent polymer modification. In this case, 2 mol% of carbon atoms on SWCNTs were reacted with the anion even as an excess of anions existed in the reaction mixture. However, the amount of grafted polymer was controlled by the molecular weight of the living polymer. In yet another case, 4-vinylpyridine monomer was anionically polymerized from n-BuLi and added to MWCNTs to prepare poly(4-vinylpyridine) grafted CNTs. The grafted polymer was able to then be converted into a polymeric ionic liquid by quaternization with bromoethane [51]. In the above cases, CNTs act as the terminating agent for living anionic polymerization. Besides that, lithiated CNTs can be used for polymer grafting by the grafting-onto approach. In this case, polymers having quenching agents on their chains, such as halogenated groups, on their chains are good candidates for grafting. Blake et al. synthesized polypropylene-grafted MWCNTs by quenching the lithiated CNTs with chlorinated polypropylene [52]. The MWCNTs obtained showed promise as high performance fillers to reinforce chlorinated polypropylene (Figure 3).
Grafting-from surface modification Carbanions obtained from CNTs by nucleophilic attack of alkyl anions are strong enough for the initiation of anionic polymerizations of various monomers, so that anionic polymerizations have been developed for covalent surface modification (grafting-from approach). As mentioned above, Viswanathan et al. reported on the polymerization of styrene from lithiated SWCNTs at the same time of the first report of CNT lithiations [24]. It is a simple method comprising lithiation of SWCNTs, followed by the addition of styrene monomer into the solution. Since anionic polymerization proceeds as a living reaction, controlling molecular weight of the grafting polymer is possible. The polystyrene modified SWCNTs are soluble in organic solvents because of the grafted polymer chains. Polyisoprene was also grafted by a similar procedure [53]. CNT/polyisoprene nanocomposites containing polyisoprene grafted SWCNTs were prepared by the polymerization of isoprene in the presence of in situ generated lithiated SWCNTs. The nanocomposites showed good solubility in organic solvents, and higher glass transition temperatures and thermal stability than that of pure polyisoprene. Polymerization of tert-butyl acrylate in THF has been initiated from the surface of SWCNTs and subsequent block copolymerization of www.sciencedirect.com
methyl methacrylate has been carried out [54]. It is noteworthy that methyl methacrylate by itself is not polymerized from the lithiated SWCNTs under the same conditions. Heteroatom-containing monomers have also been polymerized anionically for practical applications. N-Vinylcarbazol can be polymerized from the SWCNT anion, resulting in polyvinylcarbazol (PVK) grafted SWCNTs [55]. It has not only good solubility in organic solvents, but also a unique absorption in the 350–500 nm range due to a charge transfer between the electron withdrawing SWCNTs and the electron donating PVK. It is also interesting to note that it showed better optical limiting performance than that of pristine SWCNTs (Figure 4).
Conclusions After the introduction of the anionic surface modification method for CNTs, a large number of researchers have been investigating the solubilization and functionalization of CNTs, as well as the preparation of CNT/polymer composites. Just recently, it was discovered that anionic surface modification has several advantages in the variety of the reactions and the covalent modification of CNTs cause less damage on the structure than acid oxidation. As described above, most of the cases improved the solubility of the CNTs in various media to a certain extent. However, further improvement in their properties remains an ongoing challenge. Further practical applications based on the advantages of using the anionic surface modification are expected to be the main focus of research in the near term in order to make use of the unique and valuable properties of CNTs.
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